Although European Directive 1999/93/EC and the Digital Signature Act [SigG] and Signature Regulation [SigV] were designed to be completely technologically neutral, European Regulation (EU) No 910/2014 (eIDAS Regulation) prescribes, by means of Implementing Act (EU) 2015/1506, which formats must be accepted by public sector bodies for advanced and qualified electronic signatures and seals.

This chapter explains the basic cryptographic knowledge behind digital signatures in section 3.1 and the public key infrastructures based thereon in section 3.2.

The subject of this chapter is the explanation of the cryptographic basics of the digital signature in chapter 3:

• 3.1 Overview
• 3.2 Cryptographic Foundations
• 3.2.1 The principle of Digital Signatures
• 3.2.2 One-way functions and problems in number theory
• 3.2.3 Digital Signature Algorithms
• 3.2.4 Hash Functions
• 3.2.5 Post-Quantum Cryptography

Chapter 4 "Formats of Signature, Seal, Timestamp and Evidence Record" describes the formats according to "IMPLEMENTING ACT (EU) 2015/1506 - laying down specifications on formats for advanced electronic signatures and advanced seals" recognised by public authorities for electronic transactions in the internal market.